The zeolites are porous crystalline aluminosilicates that have found important applications as catalysts, adsorbents and ion exchangers. Many of these zeolitic materials have well-defined structures that form channels and cavities within them of uniform size and shape that allow certain molecules to be adsorbed, whereas they prevent other molecules, that are too large to diffuse through the pores, from passing through to the interior of the crystal. This characteristic endows these materials with molecular sieve properties. The lattice of these molecular sieves can incorporate Si and other elements of group IIIA of the periodic table, all of them in tetrahedral coordination, with the tetrahedra joined at their vertices via oxygen atoms, forming a three-dimensional network. The negative charge generated by the group IIIA elements at lattice sites is compensated by the presence of cations in the crystal, for example cations of alkali metals or alkaline earth metals. A cation of one type can be exchanged totally or partially by cations of another type by techniques of ion exchange, so that the properties of a given silicate can be varied by selecting the desired cations.
Many zeolites have been synthesized in the presence of an organic molecule that acts as a structure-controlling agent. The organic molecules that act as structure-controlling agents (SCAS) generally include nitrogen in their composition, and can give rise to organic cations that are stable in the reaction mixture.
Mobilization of silica can be accomplished in the presence of OH− groups and a basic medium, which can be introduced as the hydroxide of said SCA, for example tetrapropylammonium hydroxide in the case of zeolite ZSM-5.